1. Field of the Invention
The present invention is directed to a magnetic resonance apparatus of the type having a gradient tube at which at least one gradient coil, in which current flows in operation, is arranged and at which a number of elements for generating a force acting on the gradient tube as needed are arranged, wherein the position of the elements is selected dependent on at least one mode of characteristic vibration of the gradient tube such that this mode of characteristic vibration can be excited upon operation of the elements.
2. Description of the Prior Art
In a magnetic resonance apparatus tomograms of the subject to be examined, usually a patient, are produced through specific body planes. This occurs using electromagnetic fields. In order to enable a spatial resolution of the signal obtained in the presence of a magnetostatic basic field and an exciting radio frequency field, a gradient field is generated with a number of gradient coils. Three different gradient coils are usually utilized, generating fields in the x-y-z directions with respect to the gradient tube. Lorentz forces occur due to the flowing current, these forces acting on the gradient tube and exciting the tube to vibrate as a result of their time curve. These mechanical vibrations in turn excite the air around the gradient tube to produce fluctuations of air pressure. These fluctuations are the cause for a considerable creation of noise during the operation of a magnetic resonance apparatus, whereby noise spikes far above 100 dB occur. In order to oppose these vibrations and, consequently damp the noise, German OS 44 32 747, for example, discloses that forces be generated with piezoelectric elements that are arranged at the gradient tube, these forces opposing the Lorentz forces and thus reducing the vibrations excited by Lorentz forces. The disclosed arrangement of the piezoelectric elements, however, ensues essentially in the region of the coil conductors. The described arrangement is non-selective in view of the vibrations actually generated; a targeted damping of noise, consequently, is not possible.
The article by J. Qiu, J. Tani, xe2x80x9cVibration Control of a Cylindrical Shell Used in MRI Equipmentxe2x80x9d in Smart Mater. Struct. 4, 1995, A 75-A 81, provides a theoretical approach with respect to the arrangement of piezoelectric elements. This, however, is based on boundary conditions that are not established in practice, for which reason it leads to results that are not practically unusable. Further, Japanese Application 08-257 008 A discloses the possibility of employing piezoelectric elements for reducing noise.
An object of the present invention is to provide a magnetic resonance apparatus wherein an effective noise damping is realized.
For solving this problem, in a magnetic resonance apparatus of the type initially described, elements are inventively provided serving for the excitation of X and/or Y vibration modes which are distributed in the middle and over the circumference of the gradient tube with respect to the length of the gradient tube and/or elements serving for the excitation of Z vibration modes are distributed along the length and the circumference of the gradient tube.
The inventive arrangement of the elements is based on the fact that each vibration of the gradient tube is a superimposition of a number of vibration modes, i.e. each vibration can be reduced to specific component vibration modes. The vibration modes can make different contributions to the actual tube vibration. In the inventive apparatus, the arrangement of the elements makes it advantageously possible to intentionally and definitively excite at least one vibration mode that opposes respective component of the vibration mode of the tube vibrations and eliminates them. As a result, the tube vibration can be effectively opposed, leading to a damping thereof, and thus a damping of the generated noise as well.
The basic magnetic field proceeds along the cylinder (tube) axis; and the high currents flow within cylinder shells, for which reason the arising Lorentz forces are radially directed. The spatial distribution of the Lorentz forces is approximately symmetrical in the longitudinal direction of the tube toward the middle of the length of the tube. Given X and Y gradients, forces with opposed phase are exerted on locations lying opposite one another in the circumference direction. This means that this Lorentz force only excites characteristic vibration modes that exhibit corresponding symmetry properties. These are only those modes with uneven mode parameters, i.e. with an uneven number of equaiphase antinodes, for which reason the elements are inventively placed dependent at least on one vibration mode having uneven mode parameters. Since the elements can be inventively placed dependent on circumferential and/or longitudinal vibration modes, which is due to a desire that no significant radial vibrations within the tube occur in the acoustically relevant frequency range, it is particularly modes with uneven circumferential parameter and uneven longitudinal parameter that are relevant. The element placement can be such that only circumferential vibration modes can be generated, since it has proven that each of the characteristic modes under consideration is composed of circumferential and longitudinal vibrations, and, due to the symmetry established by the Lorentz force of the X-Y gradient, it is particularly characteristic forms having circumferential vibrations with uneven mode numbers and longitudinal vibrations that are symmetrical toward the middle of the tube that are excited. Since the suppression of only one component, i.e. either of the circumferential vibration or the longitudinal vibration, leads to the elimination or damping of the entire vibration mode and since the circumferential vibration modes can be more easily defined and separated, it suffices to select the position of the elements only on the basis of these circumferential vibration modes and to generate only such circumferential vibration modes for noise elimination. As a result of the symmetry properties of the relevant vibration modes that have already been described, the elements for generating the X and/or Y vibration modes are arranged in the middle and distributed over the circumference with respect to the length of the gradient tube. The elements thus need not be applied over the entire length of the gradient tube; on the contrary, an arrangement that is selected centrally and only at specific tube angle positions leads, due to symmetry, to an adequate damping by itself. A further advantage is that all even-numbered vibration modes exhibit an vibration node in the middle, so that a central arrangement assures that no undesired vibrations are excited during the xe2x80x9celiminationxe2x80x9d; these would in turn have a disadvantageous effect. Since the greatest unstiffened expanse in the longitudinal direction usually occur in the middle of the tube, a central positioning relative to the tube is also the most effective. As already described, it suffices for the elimination of the vibrations caused by Lorentz forces to excite only uneven circumferential vibration modes. To this end, the placement of the elements for exciting circumferential vibration modes can be selected with a mode number m=1, m=3 and, if necessary, m=5. Higher-numbered modes are not excited at the given operating frequencies, or are only excited to a negligible extent; their discrete elimination is not absolutely required.
The Z gradient, by contrast, is largely anti-symmetrical in the longitudinal direction toward the tube center. Only longitudinal vibrations that exhibit the same anti-symmetry are thus excited, i.e. points that are equidistant from the tube center in different directions vibrate with the same amplitude but with opposite phase. Consequently, only even-numbered modes are relevant here, for which reason these elements, dependent on the Z vibration modes (longitudinal vibrations), are placed to excite longitudinal vibration modes having even-numbered mode numbers. The elements are distributed along the length and the circumference of the gradient tube. Consequently, the elements serving to generate the Z vibration modes are only locally arranged distributed at selected positions along the length and the circumference, so that, as in the case of the X and Y vibration modes, the introduction of forces to generate vibrations ensues at only specific points at the tube. Since no movement occurs in the circumferential direction, the elements are arranged at the outside in the longitudinal direction. As a consequence of the anti-symmetry, they can be placed along the pipe length distributed at a number of selected locations. It is adequate to select the local arrangement for exciting longitudinal vibration modes with a mode number of l=2, l=4 and, if necessary, l=6. The elements respectively enable a quasi punctiform force generation, or introduction of force, into the tube. Each element thus makes a generation of force possible in a very small surface region of the pipe. This and the inventive placement of the elements enable the selective mode excitation. This also applies when each element for generating the required force is composed of a number of individual elements that are arranged very closely next to one another. Here, too, a quasi punctiform introduction of force ensues.
As already described, the inventively achieved noise damping is essentially based on selective and definitive excitation of vibration modes. In the ideal case, exactly one mode would be excited to oscillate with the element distribution being selected specific to that mode and all others would not be influenced. In real terms, however, the modes m=3 and m=5 are also co-excited given, for example, excitation with the circumferential vibration mode m=1. In order to be able to suppress these vibrations which are unintentionally excited, further elements can be inventively provided for generating one or more modes, these opposing these unintentionally excited modes. For symmetry reasons, only odd-numbered vibration modes (given the X and Y gradient coils) or even-numbered modes (given the Z-gradient coil) are also excited here, for which reason the arrangement of the further elements can be selected for elimination of odd-numbered and/or even-numbered modes, i.e. the further elements are arranged such that they can generate corresponding suppression vibrations. As a rule, a number of secondary modes are co-excited upon operation of the primary mode elements, for which reason the further elements can be arranged such that the mode or modes they generate simultaneously oppose two vibration modes to be eliminated. This is dependent on the xe2x80x9cprincipal modexe2x80x9d to be suppressed, particularly the vibration modes m=1, m=3, or m=1, m=5, or m=3, m=5, and l=2, l=4, or l=2, l=6, or l=4, l=6. The drive of the further elements should be such that they, in terms of their overall force influence on the gradient tube, essentially completely compensate the force influence of the first elements in view of the modes to be eliminated, i.e. the further elements are driven such that the magnitude of the force that the first elements have on the generation of the unwanted vibration modes is substantially completely compensated. The effectiveness with which a further element can compensate the force contribution of the xe2x80x9cprincipal elementxe2x80x9d is positionally dependent, i.e. an element driven the same as a xe2x80x9cprincipal elementxe2x80x9d can only compensate a specific percentage of the force. This is to be taken into consideration in the respective drive of the xe2x80x9cprincipal elementsxe2x80x9d and the further elements. The elements, and if necessary the further elements as well, can be inventively arranged with their influencing direction in the circumferential and/or longitudinal directions of the gradient tube. The respective attachment is selected dependent on the vibration mode to be generated. For example, a mode m=1 cannot be generated by elements arranged in the circumferential direction since, given this mode, no deformation in the circumferential direction occurs and thus no expansion in the circumferential direction occurs. This mode can only be adequately eliminated with elements arranged in the longitudinal direction. The advantage of such an attachment is also that all other vibration modes are excited with such an alignment and with adequate coupling, so that attachment in the longitudinal alignment is most expedient. The elements, and possibly the further elements, can be inventively arranged at the outside of the tube and/or inside of the tube. The outside arrangement has proven particularly expedient since the circumferential vibration m=1 that supplies a significant contribution to the noise creation can only be generated, and thus eliminated, given outside arrangement.
The elements, and possibly the further elements, should be respectively arranged paired, with elements in a pair lying opposite one another at the gradient tube and should be capable of being operated with opposite phase, this condition applying both to the elements acting in the circumferential direction as well as in the longitudinal direction. Expediently, the first elements can be arranged at the gradient tube at 0xc2x0 and 180xc2x0 relative to the direction of the gradient axis in question, and the further elements for eliminating the circumferential vibration modes m=3 and m=5 at xc2x145xc2x0 and xc2x1135xc2x0 given an excitation of the mode m=1, and the further elements for eliminating the circumferential vibration modes m=1 and m=5 at xc2x160xc2x0 and xc2x1120xc2x0 with respect to the tube cross-section given excitation of the mode m=3. These specific arrangements permit an adequate elimination of the odd-numbered vibration modes that are respectively unintentionally generated; the respective force components of the generating elements can be nearly completely compensated given the corresponding angular arrangement and the paired elements.
Compared thereto, the elements for exciting the longitudinal vibration mode l=2 can be arranged at {fraction (2/6)} and {fraction (4/6)} of the tube length and the elements for eliminating the longitudinal vibration modes l=4 and l=6 can be arranged at ⅙ and ⅚ of the length of the gradient tube, and the elements for exciting the longitudinal vibration mode l=4 can be arranged at {fraction (4/10)} and {fraction (6/10)} of the tube length and the elements for eliminating the longitudinal vibration modes l=2 and l=1 can be arranged at {fraction (2/10)} and {fraction (8/10)} of the tube length. As described, each tube vibration mode can be reduced to a superimposition of modes. The relationship of the vibration-forming modes, however, changes with the frequency of the influencing Lorentz force, which is in turn dependent on the operating frequency of the gradient coil. In order to take this into account, in an embodiment of the invention the elements, and possibly the further elements, can be operated such that the force they exert changes dependent on the operating frequency of the gradient coil, and thus on the generated Lorentz force. It is thereby assured that the generated xe2x80x9canti-vibrationxe2x80x9d and the xe2x80x9celimination vibrationxe2x80x9d are adequate and neither too strong nor too weak in order to achieve an optimum degree of opposing effect. With respect to the Z gradient, this frequency-dependent drive makes it possible to arrange the elements for exciting the longitudinal vibration modes only in two positions symmetrical relative to the tube center at which none of the relevant longitudinal vibrations exhibit a vibration node. These elements that are preferably symmetrically distributed around the circumference and are identically driven. A number, particularly more than six, are provided in order to avoid an unintended excitation of circumferential vibrations. As a result of the anti-symmetry and taking the element-specific force influence on the respective vibration mode into consideration, an adequate vibration generation and elimination is thus possible over a considerable frequency range. The elements themselves can be piezoelectric elements that are appropriately designed in view of the stressing, preferably as a stack of a number of elements.
In general each element can be composed of a number of individual elements that are arranged at or in the immediate environment of the predetermined tube position. In any case, both for employment of only a single element at a tube position or given employment of a plurality of individual elements at a pipe position lying in close proximity to one another, a quasi punctiform force generation or force introduction into the gradient tube occurs, i.e. a force generation or force introduction that is very limited in terms of surface area.